In Pokémon Black and White, as in most games, some wild Pokémon have a chance of holding an item. This page lists them. In this game, rare items have a 5% chance of being held by the Pokémon species listed, common items have a 50% chance. If the same item shows up in both columns, the chances are combined. Items can be obtained by using the moves Thief or Covet or by catching the Pokémon. Certain abilities can increase the chance of the wild Pokémon holding an item.

They're not useless, they just have specific uses. Lagging tail is a good item to use trick with, switching it with your opponent's item. You get their item, and they get an item that makes the slow. Star pieces are good to sell for money, and Honey is used like sweet-scent to lure out wild Pokemon to you.